1. Field of the Invention
This invention relates to filled polymers, and more particularly to highly-filled polyolefin composites, and processes for making these composites, and formed objects therefrom.
2. Description of the Prior Art
The utility of organic polymers has been broadened in recent years to the degree that rigid polymers such as the nylons, ABS (acrylonitrile/butadiene/styrene) and polyacetal resins have begun to replace the more conventional metal, wood and ceramic materials. The lower cost of polyolefins such as polyethylene could make them candidates for a wider range of applications if certain properties such as heat deflection temperature, stiffness and hardness could be improved.
Much research has been directed toward methods of improving these properties, mostly by filling polyolefins with finely-divided solids or fibrous fillers. The addition of fillers to polyolefins also serves to extend the use of polyolefins.
One method of preparing a filled polyolefin is by melt blending the polyolefin and filler. This procedure, however, requires that the polyolefin have a relatively low molecular weight, for example, an inherent viscosity of about 1 or less. While the stiffness of the resulting products is generally increased, these products typically suffer from the disadvantages of having a low elongation, e.g., less than about 15%, and being brittle, e.g., having a 0.degree. F. Izod impact strength below about 0.75 ft lb/in of notch. Moreover, problems are encountered during compounding of these products, including large power requirements for mixing machinery, degradation of the polymer by heat, nonuniformity of the filler dispersion, and poor adhesion of polymer to filler, even when "coupling compounds" are employed. These problems become more serious as the molecular weight of the polyolefin increases.
In order to avoid the problems encountered with these melt blended products, varius attempts have been made to prepare filled polyolefins without compounding the polyolefin and the filler, the most widely used such method being polymerization of the olefin in the presence of selected fillers. Most of the suggested methods involve the use of coordination catalysts. These well-known catalysts are combinations of a compound of a transition metal of Group IVa, Va or VIa of the Periodic Table and a reducing compound, for example, an alkylaluminum compound or, more broadly, an organometallic compound of a metal of Group Ia, IIa, or IIIb of the Periodic Table. The Periodic Table referred to throughout this specification is that published in Advanced Inorganic Chemistry by Cotton and Wilkinson, third edition (1972), Interscience Publishers.
These methods of forming polyolefin in the presence of filler have lead to heterogeneous products and, in general, have not provided the desired toughness for highly-filled polyolefin compositions. Although 0.degree. F. Izod impact strengths as high as about 1 ft lb/in of notch and elongations at break as high as about 50% may be obtained at 30% filler in some cases, these values drop off significantly at higher filler contents. For example, at about 50% filler the 0.degree. F. Izod impact strength of these same compositions would be of the order of only about 0.1 ft lb/in of notch and the elongation at break would be of the order of only about 1%.